1. Field of the Invention
This invention relates to a magnetic recording and reproducing apparatus having a high signal-to-noise ratio and capable of performing a recorded data reading operation at high speed.
2. Description of the Related Art
First, the known technical idea of a hard disk drive (HDD) composed of a bit-patterned medium and a spin-torque oscillator is described below. A bit-patterned medium is one type of magnetic disk of a constant density recording (CDR) system. The spin-torque oscillator is used as a magnetic read head. The recorded data is read by detecting a frequency modulation of the spin-torque oscillator. This HDD is a desirable technical concept as an HDD having high performance such as a recording density of 1 Tbit/inch2 or more and a read speed of 1 Gbit/s or more.
It is predicted that the recording density of the HDD will reach 1 Tbit/inch2 by 2012. In order to achieve the high recording density, a discrete track medium is provided in which a non-magnetic layer is embedded between respective tracks of a magnetic disk to reduce the magnetic influence of the adjacent track as is disclosed in Nikkei Electronics Nov. 19, 2007, pp 89-94. However, it is considered that a problem of thermal fluctuation can not be solved by the discrete track medium if the recording density becomes 1 Tbit/inch2 or more. As the next-generation medium technology for solving the above problem, much attention is paid to a bit-patterned medium. The bit-patterned medium is a magnetic recording medium in which minute magnetic dots are regularly arranged on the disk surface and ‘0’ or ‘1’ is written to each magnetic dot and thus the high recording density can be attained while suppressing occurrence of thermal fluctuation. Therefore, the bit-patterned medium is considered as a desirable future recording medium. The bit-patterned medium can be regarded as one type of the CDR magnetic disk which has the same recording density in the inner and outer peripheral portion.
As the recording density is increased to 1 Tbit/inch2 or more, it is necessary to further increase the sensitivity of the magnetic read head. In order to increase the sensitivity, CPP-GMR or TMR devices with high magnetoresistive (MR) ratio are actively studied. As the MR device that has received much attention in recent years, there is a MgO-based TMR device. In the magnetic head using a CPP-GMR or TMR device, the magnetization direction of the free magnetic layer is rotated according to the magnetic field from the magnetic recording medium and makes a relative angle with the magnetization direction of the fixed magnetic layer. Then, via the MR effect, the device resistance varies depending on the relative angle of the two magnetizations. In the magnetic head using a CPP-GMR or TMR device, magnetic recorded data is read by the detection of the resistance variation or voltage variation based on the MR effect. One of the noise sources in the process of reading data is thermal fluctuation of the magnetization of the free layer. The noise due to the thermal fluctuation cannot be basically solved by increasing the MR ratio of the device because the noise level is increased as the MR ratio is increased. Therefore, a new magnetic head technology is desired. In JP-A 2006-286855 (KOKAI), a magnetic head having a spin-torque oscillator is disclosed as means for solving a problem of thermal fluctuation of the magnetization of the free magnetic layer in the magnetic head. The basic structure of the spin-torque oscillator is similar to the CPP-GMR or TMR device and contains at least one free magnetic layer. In the spin-torque oscillator, the magnetization of the free magnetic layer steadily oscillates when a current with the current density of approximately 107 A/cm2 is passed through the device. Then, oscillation voltage corresponding to oscillation of magnetization is emitted from the device via the MR effect. In the spin-torque oscillator, the magnitude of fluctuation of the free layer magnetization decreases since the thermally fluctuated magnetization is forcedly subjected to steady oscillation. Therefore, the thermal fluctuation of the free layer magnetization that is a noise source in the magnetic read head using a CPP-GMR or TMR device is greatly suppressed in the magnetic read head using the spin-torque oscillator.
There are two systems of the magnetic read head using the spin-torque oscillator: an amplitude reading system and frequency reading system. In the amplitude reading system, the amplitude variation of oscillation voltage emitted from the spin-torque oscillator is used. Magnetic recorded data on the medium is obtained by detecting the amplitude variation. On the other hand, in the frequency reading system, a phenomenon that the oscillation frequency of the spin-torque oscillator changes according to the magnetic field from the magnetic recording medium is utilized. Magnetic recorded data on the medium is obtained by detecting the oscillation frequency variation. According to the above classification of the systems of a magnetic read head, the system used in the conventional magnetic head using a CPP-GMR or TMR device is classified as the amplitude reading system.
If the amplitude reading system and frequency reading system in the magnetic read head using a spin-torque oscillator are compared with each other, the latter system is more suitable for high-speed reading than the former system as is proposed in Japanese Patent Application No. 2007-249650 filed Sep. 26, 2007. The read speed in the amplitude reading system is physically limited by relaxation time of the free layer magnetization. The relaxation ratio of the free layer magnetization is roughly estimated as follows:
            180      ⁢      °              Δ      ⁢                          ⁢      θ        ×                    α        ⁢                                  ⁢        γ        ⁢                                  ⁢                  H          eff                            2        ⁢                                  ⁢        π              ⁢                  [          1      sec        ]  
where Δθ [degrees] indicates a rotational angle of the free layer magnetization by a magnetic field from the magnetic recording medium, α a Gilbert damping constant, γ a gyromagnetic ratio and Heff an effective magnetic field steadily sensed by the free layer magnetization. If Δθ=5°, α=0.01, γ=1.76×107 [1/Oe·s] and Heff=1000 Oe, the relaxation ratio can be estimated as approximately 1 GHz. That is, the limit of the read speed of the amplitude reading system is approximately 1 Gbit/s. In the frequency reading system, the read speed is not limited by the relaxation time of magnetization. Therefore, the read speed of the magnetic head of the frequency reading system using the spin-torque oscillator is expected to have a better performance than that of the existing magnetic head.
As described above, the bit-patterned medium and the magnetic read head of the frequency-change detection system using the spin-torque oscillator respectively have advantages, and an HDD with both of them can have high performance in the form of a recording density of 1 Tbit/inch2 or more and a read speed of 1 Gbit/s or more.
However, in an HDD with the magnetic read head of the frequency-change detection system using the spin-torque oscillator, a problem that does not appear in the existing magnetic head is revealed. There occurs a problem of phase noise of the oscillation voltage resulting in the reduction of the signal-to-noise (SN) ratio. In the magnetic head of the frequency-change detection system using the spin-torque oscillator, the thermal fluctuation of magnetization is largely suppressed in comparison with the existing magnetic head. However, slight magnetization fluctuation remains and appears as phase noise of oscillation voltage. A read signal output is degraded by the phase noise. In the HDD having the bit-patterned medium and the magnetic head of the frequency-change detection system using the spin-torque oscillator, it is required to output a read signal at a high signal-to-noise ratio and output a signal with reduced noise even if phase noise is present.
As described above, in the magnetic recording and reproducing apparatus having the magnetic read head of the frequency-change detection system using the spin-torque oscillator, there occurs a problem that a read signal output from the spin-torque oscillator is degraded by the phase noise.